I believe [the triangle tags] are supposed to be markers left by the Engineers as they work in searching the city. There might be some reference to them in one of the Halo books when it mentioned Engineers and their search of another human city for Forerunner artifacts.

However, there are some interesting ones which are explained by the plot of both the main game and the audio logs as time goes on. There are triangular arrows by a large circle in one room, triangles as you say by the dead Elites and triangles featureing the SuperIntendent's emblem.

My guesses: The arrows by the large circle are ships in orbit of either earth or a Halo ring. the triangles by the Elites are some kind of 'Rest in Peace' style momorial by the Engineers to those that are now also fighting the Covenant. The ones featuring the SI are of course, reference to the one free Engineer who is interfacing with the SI and probably relaying some of that information back to his fellow Engineers.

Do Engineers mourn Elites? Can you sell Engineer song like whale song?

All I can say is, I'll be in my bunk. With Romeo. He needs some TLC. Wake me if you need me . . .